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1D and 2D analyses of the IFA-610 lift-off experiments with the fuel code ALCYONE

Abstract : This paper presents finite elements analyses of the lift-off experiments performed in the HALDEN reactor: IFA-610.3/.5 for UO$_2$-Zy4 fuel rods, IFA-610.2/.4 and IFA610.7 for MOX-Zy4 fuel rods. The 1D and 2D($r$,$\theta$) schemes of the multi-dimensional fuel performance code ALCYONE are both used to study the overpressure conditions leading to the onset of temperature increase in the experiments. The 1D scheme is based on a rather standard axisymmetric description of the complete fuel rod discretized axially in slices. The 2D($r$,$\theta$) scheme allows one to study the plane strain thermo-mechanical behaviour of a pellet fragment (usually 1/8 th of the complete pellet) and its contact with the overlying cladding bore. It accounts explicitly for the additional free surface associated to pellet radial fractures and provides an estimation of the evolving pellet crack opening during loading sequences. In the proposed application to lift-off experiments, the impact of overpressure applied on the radial pellet crack borders has been studied. In the first part of this paper, the main features of ALCYONE 1D and 2D($r$,$\theta$) modelling schemes are presented. In the second part, simulations of the lift-off experiments performed with the ALCYONE 1.4 release are presented (in particular this release allows changing the nature of the filling gas in order to assess its impact on the fuel thermal behaviour). Generally, for the lift-off experiments simulated with ALCYONE code 1D scheme, a rather good agreement is obtained between predicted and measured temperature evolutions and rod axial elongations, especially when a clad-pellet bonding hypothesis is retained. Since the same material models are used in 1D and 2D, a good agreement with the measured temperature is also obtained from the 2D simulations. It is however shown that the application of the overpressure on the radial pellet crack borders has a strong impact on the onset of pellet-clad gap reopening. The resulting tangential stressing of the pellet fragment leads to radial fuel creep which tends to increase the external radius of the pellet and hence delay reopening with respect to 1D simulations results. The "mechanical lift-off" is thus better estimated when the pellet fragmentation is considered in the simulations.
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C. Bassi, J. Sercombe, B. Petitprez. 1D and 2D analyses of the IFA-610 lift-off experiments with the fuel code ALCYONE. EHPG 2014 - the 41st in the series of Enlarged Halden Programme Group meetings, Sep 2014, RØros, Norway. ⟨cea-02874737⟩

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